When British prisoner of war Robert Campbell asked the Kaiser if he could visit his dying mother, he was astonished to be given permission – on condition that he promised to return.

The Army captain kept his word and returned to the German camp after the two-week trip in November 1916, remaining in captivity until the end of the First World War.

Historian Richard van Emden, who discovered the incredible incident, said such an act of chivalry was rare even a century ago. ‘Capt Campbell was an officer and he made a promise on his honour to go back,’ he said. ‘Had he not turned up there would not have been any retribution on any other prisoners.

‘What I think is more amazing is that the British Army let him go back to Germany. The British could have said to him, “You’re not going back, you’re going to stay here”.’

Capt Campbell, who joined the Army in 1903, was leading the 1st Bn East Surrey Regiment when his battalion took up a position on the Mons-Condé canal in north-west France just weeks after war broke out in July 1914.

A week later, his troops were attacked by the German forces and Capt Campbell was seriously injured and captured. The 29-year-old was treated in a military hospital in Cologne before being sent to the prisoner-of-war camp in Magdeburg.

In 1916, he received word from home that his mother Louise was dying of cancer. He wrote to Kaiser Wilhelm II, begging to be allowed to see her one last time. The Kaiser gave him two weeks’ compassionate leave, including two days travelling in each direction by boat and train, on the proviso Capt Campbell gave his word as a British Army officer that he would return.

Capt Campbell reached his mother’s bedside in Gravesend, Kent, on November 7 and spent a week with her before keeping his promise and returning to Germany. His mother died three months later in February 1917.

Mr van Emden, 48, discovered the amazing story after reading correspondence between the Foreign Office and their German counterparts and it is told in his new book, Meeting the Enemy: The Human Face of the Great War.

He said of Capt Campbell’s amazing story: ‘I think it is such a unique example that I don’t think you can draw any parallels. In my experience, this is a one-off and is one of those things that just tickles your fancy.’

After the war, Capt Campbell was released and returned to Britain where he served in the military until retiring in 1925.

However, he rejoined his regiment in 1939 on the outbreak of the Second World War, serving as the Chief Observer of the Royal Observer Corps on the Isle of Wight. He survived that war unscathed and died in Britain in July 1966 aged 81.

Mr van Emden’s book charts the personal contacts between Britons and Germans and their feelings towards each other as the First World War progressed.

The highest display of respect he discovered was between pilots fighting above the lines. The pilots did not carry parachutes because they were too bulky for the narrow cockpits of the day.

If their aircraft caught fire, they faced the choice of burning alive or jumping out.

German pilots made it a habit to find their victims, dead or alive. If dead, they sent details of their names and burial sites across British lines. If found alive, they would invite them to a slap-up meal in their mess.

Both sides were ruthless when fighting each other in the air but observed the rules of chivalry on the ground.